Add `run_on_destroy` to `coder_script`

While there `run_on_start` and `run_on_stop` form a nice balance, some `run_on_start` scripts are actually meant to be balanced against a `run_on_destroy` trigger. As an example, using the `run_on_start` scripts to do a `git clone` of packages for the workspace, you naturally would not want to delete those when the workspace is stopped, however you might want to delete them when the workspace is destroyed.

A `git clone` is of course not a perfect example as we have terraform modules to do that for us already, and furthermore most providers are relying upon the containerization/virtualization provider to do cleanup when workspaces are destroyed, but if there are persistent resources being managed by `coder_script` snippets, it would be incredibly handy to have a `run_on_destroy` trigger to manage their lifecycles properly.

In my case, this is because I am using Coder to provide access directly to machines that have already been provisioned, and I use `remote-exec` to SSH into the devices and install the `coder` agent, then `coder_script` snippets to set up all kinds of state. I would like to clean that up properly upon workspace destruction.